Men and women show different incidence and patterns of obesity, which is a major risk factor for diabetes, cardiovascular disease, and other metabolic and reproductive diseases. This project aims to understand the biological origins of these sex differences. We will use the novel mouse model, the four core genotypes (FCG), which includes mice with testes that have XX or XY sex chromosomes, and mice with ovaries that also have either XX or XY sex chromosomes. Thus, the FCG model offers significant advantages for discriminating among several classes of biological factors that lead to sex differences, including organizational and activational effects of gonadal hormones, and direct effects of X and Y genes that create an inherent sex bias in the function of XX and XY cells. We propose to use the FCG model to tease apart the effects of sex hormones (testicular or ovarian secretions) vs. sex chromosomes (XX vs. XY genotype) on energy balance, adipose tissue function, glucose and lipid homeostasis, and other aspects of metabolism. Physiological variables will be measured during manipulations of gonadal hormonal levels after gonadectomy and hormone replacement. The X-linked genes that cause sex chromosome effects on obesity will be identified by linkage studies, gene expression, and analysis of transgenic mice. The results will shed light on fundamental sex differences in obesity and metabolic disease, leading to greater understanding of sex-specific factors that ameliorate or exacerbate disease.